A conventional variable valve timing system is disclosed in Japanese Patent Publication Laid-Open Publication No. 6(1994)-10625. With reference to FIG. 4, this conventional system includes a timing pulley 101 which is driven by a crank pulley 102 of an engine 103 via a belt 104. A cup-shaped cover 111 is fixed to the pulley 101 and an open portion of the cover 111 is closed by the pulley 101 to form an inner space 112 therein. An inner cylindrical surface 101a of the pulley 101 is rotatably supported on a cam shaft 121. Several cams 124 (only one cam is shown in FIG. 4) are fixed on the shaft 121 and each of them drives an intake valve 125 (or an exhaust valve) against the force of a valve spring 126.
The conventional system also includes a cup-shaped case 113 that is fixed to one end of the cam shaft 121 and divides the inner space 112 into a damper space 122 and an oil pressure space 123. A cylindrical piston 114 is located between the pulley 101 and the case 113 to transmit the rotational torque of the pulley 101 to the cam shaft 121. Helical connections such as a helical spline are individually formed between the pulley 101 and the piston 114 and between the piston 114 and the case 113.
When the piston 114 is moved in the axial direction in response to pressure in the oil pressure area 123, the cam shaft 121 rotates relative to the pulley 101 according to the action of the helical connections. As a result, the angular position of the cam shaft 121 advances with respect to the angular position of the pulley 101.
The cup-shaped cover 111 is fixed to the timing pulley 101 and rotates relative to the cup-shaped case 113. A cylindrical gap 131 is formed between the inner cylindrical surface of the cover 111 and the outer cylindrical surface of the case 113 in the damper region 122. A portion 132 is formed at the left end of the cylindrical gap 131 in the damper region 122 and is in fluid communication with the cylindrical gap 131. Viscous fluid is enclosed in the damper space 122 and the gap 131 is also filled with the viscous fluid.
When the engine 103 is in operation, the cam shaft 121 rotates in a uniform direction (in a positive direction). However, torque arising from the urging force of the valve spring 126 imparts an undesirable positive and negative rotation to the cam shaft 121. The undesirable rotation is transmitted to the case 113, and the case 113 rotates relative to the cover 111, the piston 114 and the pulley 101. Therefore, hitting or impact noises occur in the helical connections because the helical splines of the helical connections have backlash.
To prevent such noise, viscous fluid in the cylindrical gap 131 is designed to retard relative rotation. That is, the viscosity of the viscous fluid resists the relative rotation between the case 113 and the cover 111. However, the viscous fluid in the cylindrical gap 131 flows out to the portion 132 since fluid pressure in the gap 131 is higher than that in the portion 132. As a result, the amount of viscous fluid in the gap 131 is lessened and the retarding action thereof is reduced.
Damper mechanisms such as those described in U.S. Pat. No. 5,067,450 (issued on Nov. 26, 1991) and U.S. Pat. No. 5,090,365 (issued on Feb. 25, 1992) have a more powerful damper action which may overcome the aforementioned drawbacks. That is because each of them has a labyrinth mechanism between the timing pulley and the cup-shaped case. However, such a mechanism is complicated in structure and is relatively high in cost.